LinuxQuestions.org - NetApp SAN Issuu

Quote:

Linux, by default, places detected disk volumes under /dev/ with root:disk as user/group (for example, /dev/sdq) and not read/writeable for anyone else than root user/group. So Oracle cannot handle this as it typically does not run under root id. Also, if ASM scans /dev/ it will have to scan a lot of devices because /dev/ is home to not just all disks, but also all other block (and character) devices.
So we need to present ASM volumes under a path that matches the ASM diskstring and has different userid/group and permissions. One way of doing this is using the “mknod” command to create references (inodes) to the disks under /dev. So for example /dev/sdq and /dev/oracleasm/disks/myvol both point to a Linux device with major ID 8 and minor id 80. They are essentially different names for the very same thing and can have different permissions (so /dev/sdq can have root:root and myvol can have oracle:asm as ownership and still being the same device). The problem is that a) it’s a very manual intensive process, b) it’s error prone and c) not guaranteed to be consistent across reboots.

ASMlib

Oracle therefore created a tool called ASMLib that consisted of a custom kernel module and a set of command line tools. The kernel module (driven by the CLI) would scan all devices in /dev/* after boot and if it found the right signatures it would clone those (by creating additional device inodes using mknod) typically using diskstring /dev/oracleasm/disks.